As is known, for control of the electro-injectors of a common rail injection system, each electro-injector is habitually supplied with a current, the development of which over a period of time comprises a section of rapid increase to a first maintenance value, a first section of amplitude which oscillates around the first maintenance value, a first section of decrease to a second maintenance value, a second section of amplitude which oscillates around a second maintenance value, and a second section of rapid decrease to a value of approximately zero.
In fact, as is known, an electro-injector comprises an outer body defining a cavity which communicates with the exterior via an injection nozzle, and in which there is accommodated a pin which is mobile axially in order to open and close the nozzle, under the opposite axial thrusts of the pressure of the fuel injected on the one hand, and of a spring and a rod on the other hand, which rod is disposed along the axis of the pin, on the side opposite the nozzle, and is activated by an electro-magnetically controlled metering valve.
In the initial stage of opening of the electro-injector, it is necessary not only to apply considerable force against the action of the spring, but also the rod must be moved from the position of rest to the position of activation in the shortest possible time. For this reason, the excitation current for the electro-magnet in the first stage is somewhat high (first maintenance value). The rapid increase in the development of the current to the first maintenance value is necessary in order to guarantee sufficient temporal precision at the moment of initiation of the activation. However, once the rod has reached the final position, the electro-injector remains open even with currents which are less high, such as the sections of decrease and maintenance around the second maintenance value in the development of the excitation current of the electro-magnet.
In order to obtain this development of the excitation current, use was previously made of a control device in which the electro-injectors were connected firstly directly to a supply line, and secondly to a ground line, via a controlled electronic switch.
However, this control device had the disadvantage that any short-circuit to ground of one of the terminals of any of the electro-injectors, caused for example by a loss of insulation in a wiring conductor of the electro-injectors themselves and by contact of this conductor with the bodywork of the motor vehicle, gave rise to definitive damage to the electro-injector itself and/or to the control device, thus causing the vehicle engine to cut out, which is a decidedly dangerous situation when travelling.
In order to eliminate this dangerous disadvantage, European patent EP 0 924 589 in the name of the applicant proposed a control device in which the electro-injectors are floating in relation to the supply line, i.e. they are connected to the supply line and to the ground line via respective controlled electronic switches, which are general produced by means of MOSFET transistors. By this means, any short-circuit to ground or to the supply of one of the terminals of the electro-injectors does not cause damage to the control device, and therefore make the vehicle engine cut out, but simply leads to non-use of that individual electro-injector, thus making it possible to continue travelling with one electro-injector less.
In particular, in the control device described in the said patent, the presence of a short-circuit to ground or to the supply of one of the terminals of the electro-injectors is determined on the basis of the voltages which are present at the terminals of the electro-injectors themselves.
In detail, to each of the terminals of the electro-injectors there is connected a CMOS logic gate with hysteresis, for example a logic inverter, which supplies as output a feedback logic signal which assumes a low logic level when the voltage which is present at the corresponding terminal is higher than the upper threshold voltage of the CMOS logic gate itself, and a high logic level when the voltage which is present at the corresponding terminal is lower than the lower threshold voltage of the CMOS logic gate.
The presence of faults at the terminals of the electro-injectors is thus determined by detecting any incompatibilities between the values of the control logic signals of the controlled electronic switches which connect the electro-injectors to the supply line and to the ground line, and the feedback logic signals generated by the CMOS logic gates connected to the terminals of the electro-injectors.
If a fault is detected, activation of the electro-injector at which this fault has occurred is immediately interrupted by opening the controlled electronic switches which connect it to the supply line and to the ground line, such as to disconnect it physically from the remainder of the control device and therefore allow the engine to continue to run, even if with a reduced number of cylinders and reduced performance levels.
Although extensively used, this type of protection of the electro-injectors, against short-circuits to ground or to the supply of their terminals, has some disadvantages which do not permit adequate use of all their merits.
In particular, if there is a short-circuit to ground of the terminal of an electro-injector which is connected to the supply line, when the controlled electronic switch which connects the terminal to this supply line is closed, this short-circuit is not detected in good time, and a very high current flows into the controlled electronic switch.
In fact, in the above-described circuit topology, in order to detect the presence of this short-circuit to ground, the voltage of the terminal of the electro-injector which is connected to the supply line should drop below the lower threshold voltage of the CMOS logic gate connected to this terminal, such as to trigger the gate and thus switch the feedback logic signal supplied by the latter.
In reality however, even in the presence of a short-circuit to ground, the voltage of the terminal of the electro-injector which is connected to the supply line is unlikely to drop below the threshold indicated. In fact, in order for this to take place, the voltage at the ends of the controlled electronic switch which is connected to the supply line must assume a value equivalent to about 10 V (battery voltage, which is generally approximately 12 V, less the lower threshold voltage of the CMOS logic gate, which is generally approximately 2 V), and since the resistance of a controlled electronic switch is usually in the order of tens of mΩ a current of several hundred amps must flow in this switch.
However, this situation cannot arise since it is not possible have such strong short-circuits, i.e. such low short-circuit resistances, as to permit passage of a current with this value. Consequently, in the event of a short-circuit to ground of an electro-injector which is connected to the supply line, the voltage of this terminal drops, but not to the extent that it falls below the lower threshold voltage of the CMOS logic gate.
The consequence of this is that, in addition to the failure to detect the fault, there is also passage of very high short-circuit currents which are not detected by the CMOS logic gates, and can therefore damage the control device of the electro-injectors, or, in the best of hypotheses, which can supply it with incorrect diagnostic information induced by the electric noise generated by the short-circuit current itself.
The object of the present invention is thus to provide a control device for electro-actuators which is free from the above-described disadvantages.